Problem: Solve for $r$ : $-9 = 11 + r$
Solution: Subtract $11$ from both sides: $ \begin{eqnarray} \\ \hphantom{-9 {- 11}} &\hphantom{=}& \hphantom{r} \\ -9 &=& 11 + r \\ \\ {-11} && {-11} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ -9 {- 11} &=& r \\ \end{eqnarray} $ Simplify. $ r = -20$